Vehicle battery packs can be used to fully or partially power certain vehicles, like plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), battery electric vehicles (BEV), etc. The vehicle battery pack may consist of a number of individual cells, where the number can range from tens to hundreds of cells, for example. In vehicle battery packs having a multitude of cells, discrepancies from one cell to another can exist. These discrepancies may involve the state of charge (SOC), voltage, current and/or other characteristics of the cells, and may be caused by a whole host of different factors.
Discrepancies in SOC, for example, can lead to over- and/or undercharging certain battery cells within the vehicle battery pack. Consider the case where a battery pack includes a cell that exhibits a 20% higher SOC value than the average cell within the battery pack. If all of the battery cells are subject to the same charge cycle and all are charged according to the average SOC value, then the outlier cell with the 20% higher charge is likely to be overcharged. In certain battery types like lithium-ion batteries, there can be a preferred charge range (e.g., 30-70% SOC) where overcharging can damage the cells and in turn impact the overall performance of the vehicle battery pack. A similar situation can occur with undercharged cells and a discharge cycle. Replacing a damaged or failed cell can be costly and may not resolve the issue since the characteristics of the new cell may differ from those of the used or aged cells; thus, the over- and/or undercharging problem may still persist. Replacement of the entire vehicle battery pack is likely to be costly and undesirable.
One approach for addressing discrepancies between the battery cells involves a process called ‘cell balancing’ or ‘cell equalization’, where the process attempts to balance or equalize the level of charge on the different battery cells within the vehicle battery pack. In order to do this, the cell balancing process needs to accurately evaluate the characteristics of each battery cell; a process that can be made difficult by phenomena such as double layer effect, ohmic resistance, diffusion and hysteresis.